170 Dr. J. Kerr on Reflection of Polarized Light 



ority of reversal to break, and even of make to break, in the 

 case of steel, than in the case of soft iron (7) ; but I do not 

 know how far my judgment in this matter may have been in- 

 fluenced by expectation. 



13. Synopsis of the preceding Results — two Laws with two 

 Exceptions. 



First Laic. The right-handed current conspires with a small 

 right-handed rotation of the analyzer from extinction ; and so 

 forward. 



Second Law. The right-handed current conspires with a 

 small left -handed rotation of the polarizer from extinction ; and 

 so forward. 



First Exception. When the plane of polarization of the light 

 incident on the mirror is perpendicular to the plane of inci- 

 dence, the First Law is reversed for all incidences between 

 principal and normal. 



Second Exception. When the plane of polarization of the 

 light incident on the mirror is parallel to the plane of incidence, 

 the Second Law is reversed for all incidences between principal 

 and normal. 



14. I may say here, with reference to the various statements 

 of fact which I have made in the preceding articles, that they 

 are founded on a large number of perfectly concordant obser- 

 vations. The angles of incidence quoted are of course only 

 rough approximations (2); and some of the other details may 

 be modified by future observation ; but the broad facts are as 

 certain to my mind as any thing in Physics. 



The next two experiments afford interesting verifications of 

 former results, as they show that when two actions already 

 known as conspiring actions are applied separately, their 

 optical effects are similar, or rather similarly directed. To 

 prepare the way, I must mention some delicate optical phe- 

 nomena, which present themselves in connexion with very 

 small movements of either Nicol through the position of ex- 

 tinction. Each Nicol is supposed to be near the mirror ; 

 the piece P is in position (2); and the incidence is between 

 principal and normal. When the extinction is perfect, and the 

 observer's attention has been once directed, he can generally 

 detect an obscure cloud, which is pretty definitely formed, and 

 sometimes finely formed as a dark horizontal band, covering 

 the old place of the reflected image, and extending well across 

 the field. In these circumstances, any almost immeasurably 

 small rotation of either Mcol, in one direction or the contrary, 

 produces a regular and very sensible change in the field of 

 vision — a continuous displacement of the band, upwards in one 

 case and downwards in the other, the form and direction of 



